The present invention relates to an electrical quantity sensor.
An electric current sensor using a hall element such as that shown in FIG. 4 has hitherto been provided as an electrical quantity sensor of this type. When an electric current to be detected flows into a current path 20 passing through a core 11, a magnetic flux corresponding to the value of the electric current develops in the core 11. A hall element 12 produces a voltage corresponding to the magnetic flux. The voltage output from the hall element 12 is amplified by an amplifier 13, and the thus-amplified voltage is output to the outside.
The electric current sensor set forth detects the value of a current flowing through the current path 20 on the basis of a magnetic flux developing in the core 11 in accordance with the electric current flowing through the path 20. If a heavy current flows into the current path 20 to make the magnetic flux developing in the core 11 saturated, the current flowing through the current path 20 cannot be detected accurately.
As shown in FIG. 5, there has been proposed a current sensor based on the above-described current sensor. The sensor is provided with a current amplification circuit 14 which further amplifies the output from the amplifier 13 and outputs the thus-amplified output as a current signal. The electric current is output from the current amplification circuit 14 via a compensating winding 15 coiled around the core 11. Here, the compensating winding 15 is coiled in the direction in which the magnetic flux developing in the core 11 is neutralized.
When the current to be detected flows through the current path 20 passing through the core 11, a magnetic flux corresponding to a current value develops in the core 11. The hall element 12 produces a voltage corresponding to the magnetic flux. The voltage output from the hall element 12 is amplified by the amplifier 13, and the thus-amplified voltage is converted into a current signal by means of the current amplification circuit 14. The current signal is output via the compensating winding 15. Since the compensating winding 15 is coiled in the direction in which the magnetic flux developing in the core 11 is neutralized, occurrence of magnetic saturation of the core 11 can be prevented, thus improving the accuracy of detection of the current sensor.
However, the measurement accuracy of the current sensor greatly depends on the performance of the hall element 12. Hence, the measurement accuracy of the current sensor is greatly affected by variations in performance of the hall element 12 or the temperature characteristic of the hall element 12. The reason for this is that the temperature characteristic of the hall element is changed by package stress exerted on a balance element.
In the latter current sensor, the magnetic flux developing in the core 11 is neutralized by the magnetic flux that appears in the compensating winding 15 as a result of the current output from the current amplification circuit 14, thereby preventing magnetic saturation of the core 11. The core 11 must be wound around the compensating winding 15, thus adding to costs. In order to cancel the magnetic flux developing in the core 11, the current output from the current amplification circuit 14 must be made large, thereby resulting in an increase in current consumption.
The present invention has been conceived in light of the foregoing problem and aims at providing a low-cost, low-current-consumption current sensor which has achieved a reduction in variations in measurement accuracy.
To achieve the object, the present invention provides an electrical quantity sensor comprising: an oscillation circuit for producing a clock signal of predetermined frequency; a light-emitting element which blinks at a predetermined frequency in accordance with the clock signal output from the oscillation circuit; a switching element which is optically coupled to the light-emitting element and is brought into conduction when the light-emitting element illuminates; a transformer whose primary winding is connected, by way of the switching element, to input terminals which receive a d.c. electric signal; and a synchronization detecting circuit which synchronously detects an a.c. electric signal developing in a secondary winding of the transformer while the clock signal is taken as a reference, thereby producing a d.c. voltage signal corresponding to the amplitude of the d.c. electric signal.
When the light-emitting element blinks at a predetermined frequency corresponding to the clock signal output from the oscillation circuit, the switching element is turned on and off in accordance with blinking action of the light-emitting element. A d.c. electric signal is converted into an a.c. electric signal by means of switching action of the switching element. The thus-converted a.c. electric signal is transmitted to the secondary side of circuitry. An a.c. electric signal developing in the secondary winding of the transformer is synchronously detected by means of a synchronization detecting circuit while a clock signal is taken as a reference. As a result, the d.c. electrical signal corresponding to the amplitude of the d.c. electric signal can be obtained. In this way, the d.c. electric signal is converted into an a.c. electric signal, and the a.c. electric signal transmitted, by way of the transformer is restored into a d.c. electric signal by means of synchronous detection. In contrast with a related-art current sensor using a hall element, there is not used a hall element having great variations. There is prevented occurrence of a situation in which performance of products varies among the products, which would otherwise be caused by the performance of a hall elements serving as one constituent component of the current sensor. Thus, variations in detection accuracy and temperature characteristic of the current sensor can be reduced. A compensating winding for preventing magnetic saturation becomes obviated, and hence an increase in costs of the electrical quantity sensor can be prevented. Moreover, a circuit under detection is electrically insulated from a detection circuit by means of a light-emitting element and a switching element, which are optically coupled together, and a transformer. Hence, the influence to be exerted on the circuit under detection can be diminished. The electrical quantity sensor according to the present invention produces a d.c. voltage signal corresponding to the amplitude of a d.c. electric signal. The circuit of the electrical quantity sensor is constituted of an oscillation circuit, a switching element, an IC such as a synchronization detecting circuit, and a light-emitting element. Hence, there can be provided an electrical quantity sensor involving low current consumption.
Preferably, the d.c. electric signal is a voltage across a detection resistor inserted into an electrical path through which an electric current to be detected flows. Thus, there can be embodied a current sensor which detects a d.c. electrical current.
Preferably, the d.c. electric signal is a d.c. voltage signal. Thus, there can be embodied a voltage sensor which detects a value of d.c. voltage.
Preferably, the switching element is a MOSFET. There can be embodied an electrical quantity sensor for detecting the electrical quantity of a circuit through which an electric current flows in both directions.